Numerous types of RFID tag are commercially available. A common type of RFID tag stores a small amount of data, such as an identifying number, and transmits the data to a nearby interrogating device, when the latter issues an interrogation signal.
In general, RFID tags can be viewed as containing four primary components. Three of the components are commonly fabricated in a single integrated circuit, IC, and they are: (1) a receiver, (2) a transmitter, both of which are sometimes termed a transceiver, and (3) memory to store data, such as the ID number stated above. The fourth component is an antenna, used to communicate with the interrogator.
In some designs, the antenna can be included in the IC, or fabricated on the same silicon wafer as the IC. The antenna can also be external to the IC.
In addition, other components can be present, to perform tasks such as (1) detecting an incoming interrogation signal, and in response launching a dormant tag into operation, (2) absorbing operating power from incoming rf radiation, (3) reading data in the memory and transmitting the data to the interrogator, and (4) discriminating an address in an incoming polling signal, to discern whether the interrogation signal is addressed to the RFID tag associated with the components, or to another RFID tag.
Some RFID tags are passive. They contain no power supply, and obtain operating power from rf energy delivered by the interrogator. Other RFID tags do contain power supplies, such as batteries of the size used in hearing aids. These latter RFID tags can not only transmit stored data, but they can also receive data from the interrogator, and can write the data received to memory in the RFID tag.
In general, passive devices do not receive and store incoming data but, of course, exceptions are possible.
The frequency of rf radiation used depends on the particular application of the RFID tags. For example, some tags use low-frequency radiation, in the AM or FM radio bands, which span roughly from 0.5 MHz to 150 MHz. Such radiation can pass through buildings and other structures. Using such radiation, one can read an RFID tag through a wall or building.
At higher frequencies, such as 1,000 MHz, the radiation begins to acquire the properties of visible light. Visible light will not penetrate walls and buildings. Using such high frequencies, one can only communicate with RFID tags which are within one's line-of-sight, with no intervening obstructions.
Further, at high frequencies, the presence of nearby conductive objects can interfere with operation of the RFID tags. While the detailed mechanism of the interference is complex, one can view the conductive objects as creating “echoes” of the rf signals. The echoes can jam communication. For example, the echoes can destructively add together, forming nulls where the net signal is zero. If the RFID tag or the interrogator is located at a null, no signal will be detected.
Therefore, when RFID tags using high-frequency radiation are used in the proximity of conductive objects, such as sea water or bodies of metal, problems can arise. As a specific example, problems are found when high-frequency RFID tags are used on steel shipping containers, particularly when multiple such containers are present.